The embodiments described herein relate generally to a photovoltaic (PV) power generation system, and more specifically, to methods and systems for extracting power from a photovoltaic collection system.
Solar energy has increasingly become an attractive source of energy and has been recognized as a clean, renewable alternative form of energy. Photovoltaic (PV) cells generate direct current (DC) power with the level of DC current being dependent on solar irradiation and the level of DC voltage dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC power into AC power. Typically, PV inverters employ two stages for power processing with the first stage configured for providing a constant DC voltage and the second stage configured for converting the constant DC voltage to AC current and voltage that is compatible with the grid. Typically, the PV inverter uses a DC link as an intermediate energy storage step, which means that the PV inverter includes a DC to DC converter that converts the unstable PV array voltage to a stable DC voltage and a DC to AC inverter that subsequently converts the stable DC voltage into an AC current that can be injected onto the grid. The efficiency of the two-stage inverter is an important parameter affecting the performance of the PV system and is a multiple of the individual stage efficiencies.
In order to obtain a higher current and voltage, PV cells are electrically connected to form a PV module. In addition to a plurality of PV cells, the PV module may also include sensors, for example, an irradiance sensor, a temperature sensor, and/or a power meter. PV modules may also be connected to form a string and multiple strings may be connected to form a PV array. Typically, the DC voltages output by the PV array are provided to a grid inverter, for example, a DC to AC voltage inverter. The DC to AC voltage inverter converts the DC voltage to a single or three-phase alternating current (AC) voltage and current. The three-phase AC output can be provided to a medium voltage power transformer, which steps up the voltage to produce a three-phase medium-voltage AC that is injected into a power distribution grid.
Most PV power generation systems use a central DC to DC converter to convert the entire power output of the PV array resulting in a relatively high-cost and high-weight solution. Also, the central DC to DC converter typically uses a maximum power point tracker (MPPT) that includes sensors for measuring array voltage and current for use in computing array power. Such sensors are in addition to sensors needed to operate the DC to DC converter. Moreover, a disadvantage of a PV power generation system that includes a full-power conversion DC to DC converter is referred to as the efficiency compounding effect. The efficiency of a PV power generation system can be no higher than the efficiency of the central DC to DC converter. The total efficiency of the DC to AC conversion is reduced by 1% to 2% due to the compounding of the DC to DC converter efficiency and the DC to AC inverter efficiency. Furthermore, a fault in the full-power rated central DC to DC converter may cause a failure of the entire PV array. In order to prevent such a PV array failure, additional array series diodes and fuses are typically used to isolate a DC to DC converter failure.